Formability created by the Bauschinger effect?

This paper describes tests to determine if the Bauschinger effect can act as an\ud additional source of formability, this may possibly explain the enhanced formability observed in incremental sheet forming Small cylindrical products have been made by deep drawing and wall ironing. The upper edges have been necked to various levels of reduction. Tensile test specimens have been cut from the necked upper edges and tested, causing an almost perfect stress/strain reversal. An increase of strain could be observed indeed. The results are discussed using the Considère condition as a guideline

Tribology of flat contacts and its application in deep drawing

The difference between steel and aluminium in sheet metal forming operations, and the influence of roughness in these operations, have been studied by making deep draw and friction experiments. The friction tests have been carried out with flat contacts to simulate the conditions in the blankholder. In deep drawing the influence of roughness cannot be neglected. The punch force is affected by the roughness, and related properties such as the dimensions of the product after deep drawing and the fracture limit, are affected as well. The relative amount of this influence depends on the amount of lubricant, the type of product and the type of sheet metal, and can also vary with the state of the process. In rectangular products however the effects were less clear than in cylindrical products. For aluminium the influences of process conditions in deep drawing are stronger than for steel, caused by strong asperity flattening. When using a high speed and much lubricant at aluminium, the friction in the blankholder became so low that the blankholder force could be increased up to the limit of the press without causing fracture. Friction tests showed that for steel the friction as expressed in a Stribeck curve, barely depends on the pressure, while for aluminium the influence of pressure is strong. Both the position of the region of mixed lubrication, and the 'classic' friction coefficient at conditions of boundary lubrication are influenced by pressure. The influence of pres-sure could be linked to the phenomena encountered in the deep drawing tests success-fully. The mechanisms responsible for the generation of pressure in the lubricant at mixed lubrication have been examined. Based on these findings a model for mixed lubrication has been developed which can describe many of the observed effects including the consequence of asperity flattening

Material characterization at high strain by adapted tensile tests

The strength of materials at high strain levels has\ud been determined using the so-called Continuous-Bendingunder-\ud Tension (CBT) test. This is a modified tensile test\ud where the specimen is subjected to repetitive bending at the\ud same time. This test enables to create high levels of uniform\ud strain. A wide variety of materials has been tested this way.\ud The strength of the material after CBT testing has been\ud measured in different ways: by secondary tensile tests, by\ud interrupted CBT tests, and directly from the fracture in the\ud CBT test. All methods yield similar results: the strength is\ud largely unaffected by the cyclic pre-deformation and mainly\ud depends on the overall increase in length. Only for multiphase\ud materials the strength shows a minor influence of\ud CBT test conditions. The hardening follows the extrapolated\ud hardening observed in a conventional tensile test, except for\ud brass. This test method can potentially be used for measuring\ud hardening curves at high strain levels

Contact effects in bending affecting stress and formability

If a strip is pulled over a curved tool there is a contact stress acting on the strip. This contact stress changes the stress state in the material, which is analysed with a simple model. One effect is that the yield stress in tension is reduced. Predictions by the model agree with observation from a 90-degree bending test found in literature, and indirectly with observation from a stretch-bend test also found in literature. Another effect is that a change in stress state also affects the formability. This is analyzed by applying the maximum force condition on this situation. The predictions agree with a more thorough analysis of the effect of thickness stress in general, but the predictions of both methods are lower than actually observed in tests. There may be other mechanisms at work, and one candidate is presented

The FLC, enhanced fromavbility, and incremental sheet forming

The FLC is a well known concept in the sheet metal forming world. It is used to map the material’s formability and the make-ability of a product. The FLC is valid only within certain restrictions. These restrictions are: A: a straight strain path; B: absence of bending; C: absence of through-thickness shear; D: a condition of plane stress.\ud The formability of a material can be increased significantly if one is allowed to violate any of these restrictions, meaning either: use a complex strain path, incorporate bending, incorporate through-thickness shear, or apply a contact stress. Both shear and contact stress change the stress state, and both lower the yield stress in tension and raise the necking limit up to a certain level. Bending creates a non-uniform stress distribution over the thickness of the sheet, resulting in a reduction of the yield force in tension, and it creates a range of stable elongation depending on the sheet thickness at each passage of the punch. The effect of a complex strain path depends on the particular situation; in incremental sheet forming it is based on non-isotropic hardening.\ud In general it will not be possible to create such conditions in the entire product at once. However it is possible to do this intentionally in a small, restricted zone by creating special situations there. By moving this zone over the entire product the whole part can be made with increased formability. This technique of incremental forming is explained briefly. The special conditions around the punch indeed violate the FLC restrictions mentioned above. The enhanced formability obtained in incremental sheet forming is illustrated with many examples

Fractional behaviour at cyclic stretch-bending

The fractional behaviour at cyclic stretch-bending has been studied by performing tensile tests at long specimens that are cyclically bent at the same time, on mild steel, dual-phase steel, stainless steel, aluminium and brass. Several types of fracture are observed, these are discussed, as are the underlying mechanisms. The results agree with those obtained with 900 bending tests, concluding that the fracture will be orientated at 900 if the deformation is localized into a small transverse zone. There is a relation be-tween the formability and a certain fracture type, also microscopic examination revealed differences, but no conclusive explanation could be give

Strain in Shear, and Material Behaviour in Incremental Forming

This paper discusses some consequences of forming by shear, a situation that is sometimes claimed to occur in incremental forming. The determination of the principal strains and principal directions is discussed in detail. Two methods are presented: using a circular grid (although simulated on the computer), and by deriving formulae from the theory; both yield identical results. The strains assuming forming by shear are found to be (much) higher than in situations of forming by stretch. This affects notably more fundamental studies on material behaviour in incremental forming. The effects are illustrated using experimental data obtained with pre-stressed material

An overview of stabilizing deformation mechanisms in incremental sheet forming

In incremental sheet forming (ISF) strains can be obtained well above the forming limit curve (FLC) that is applicable to common sheet forming operations like deep drawing and stretching. This paper presents an overview of mechanisms that have been suggested to explain the enhanced formability. The difference between fracture limit and necking limit in sheet metal forming is discussed. The necking limit represents a localized geometrical instability. Localized deformation is an essential characteristic of ISF and proposed mechanisms should stabilize the localization before it leads to fracture. In literature six mechanisms are mentioned in relation to ISF: contact stress; bending-under-tension; shear; cyclic straining; geometrical inability to grow and hydrostatic stress. The first three are able to localize deformation and all but the last, are found to be able to postpone unstable growth of a neck. Hydrostatic pressure may influence the final failure, but cannot explain stability above the FLC